Do you have all the archives saved from primary? You would need those in order to bring them in sync. First, you should find what is causing the problem? What is the guarantee it won;t happen again after the rebuild? I have seen many cases where rebuilding does not fix the problem unless root cause has been resolved.

This query might tell you where it is stuck

/*** Use to check errors ***/

SET LONG 1000

ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MON-YY HH24:MI:SS';

SELECT EVENT_TIME, COMMIT_SCN, EVENT, STATUS FROM DBA_LOGSTDBY_EVENTS order by event_time;